1. Field of the Invention
The present invention relates to an apparatus and a method for measuring arterial elasticity. The apparatus according to the present invention can be used for noninvasively measuring the degree of sclerosis of an artery during diagnosis and treatment of the human body.
2. Description of the Related Arts
Recently attempts have been made to measure vascular viscoelasticity by using the relationship between the pressure and the volume of an artery. This measurement of vascular viscoelasticity, preferably the frequency characteristic of the viscoelasticity as well as the viscoelasticity itself, provides a measure of the degree of sclerosis of an artery.
There have been proposed, for example, a method of measuring arterial elasticity in human fingers using photoelectric plethysmography, and a volume compensation type method of measuring arterial elasticity in human fingers together with the frequency characteristic thereof, using photoelectric plethysmography.
In the method of measuring arterial elasticity using photoelectric plethysmography, it is assumed that Pas represents the systolic arterial pressure, Pam the mean arterial pressure, I the light intensity transmitted across the tissues, .DELTA.I the changes in transmitted light intensity, and I.sub.t the light intensity transmitted across incompressible tissues other than the arterial systems. Under the above assumptions, the volume elastic modulus E.sub.v is derived according to the following equation. EQU E.sub.v =3/2 (P.sub.as -P.sub.am)/{(.DELTA.I/I)log(I/I.sub.t)}
In the volume compensation type method of measuring arterial elasticity, together with the frequency characteristic thereof, a sinusoidal change .DELTA.P of the transmural pressure P.sub.t having a given frequency is applied to an artery of a finger of a human body, and the corresponding change .DELTA.V of the volume of the artery is measured.
In fact, an artery is not a simple elastic tube but a tube having a striking viscoelastic property. The value of the change .DELTA.V of the volume of the artery is changed, even under the same pulse pressure, due to a change of the frequency component of the waveform of the blood pressure caused by a change in the heart rate or blood pressure, and preferably the volume elastic modulus E.sub.v may be expressed is represented in association with the frequency characteristic. To facilitate an easy understanding of this, E.sub.v may be expressed as a transfer function to determine a volume change ratio .DELTA.V/V caused by a transmural pressure change .DELTA.P.
The method of measuring arterial elasticity using photoelectric plethysmography is described, for example, in A. Kawarada et al.: "Noninvasive Automatic Measurement of Arterial Elasticity in Human Fingers and Rabbit Forelegs Using Photoelectric Plethysmography", Medical & Biological Engineering & Computing, Vol. 24, No. 6, P. 591 to 596, November 1986. The volume compensation type method of measuring arterial elasticity together with the frequency characteristic thereof is described, for example, in H. Shimazu et al.: "Noninvasive Measurement of Frequency Characteristics of Arterial Elastic Modulus in Human Fingers", Proceedings of 26th Conference of Japan Society of Medical Electronics & Biological Engineering, P. 213, Apr. 1 to 3, 1987.
In the above-mentioned first method for measuring arterial elasticity, however, problems have arisen in that the correct arterial elasticity cannot be measured because of the use of an estimated value for the pulse pressure value, and in that the frequency characteristic of the arterial elasticity cannot be obtained.
Also, in the above-mentioned second method, of the volume compensation type, for measuring arterial elasticity, a problem has arisen in that the volumetric pulse wave of two fingers, such as the index finger and the third finger, is different and accordingly, the motion of the artery can be eliminated in the index finger but cannot be eliminated in the third finger, even if the same pressure change is applied to these two fingers, and thus errors occur in the measurement of the arterial elasticity.
Further, in the above-mentioned second method, i.e., the volume compensation type, of measuring arterial elasticity, it is necessary to apply a sinusoidal pressure change and to scan the frequency of the pressure change, and accordingly, a relatively long time on the order of minutes is needed to determine the frequency characteristic of the arterial elasticity.